This invention relates to a process for producing a coil of cable-in-conduit type superconductor with reduced ac loss from the induced current as produced between superconductor strands upon application of a time-varying magnetic field.
When the conventional coil of cable-in-conduit type superconductor is produced by the wind-and-react technique, it is necessary to ensure that superconductivity characteristics (e.g. critical current) will not deteriorate after heat treatment and to this end, the smallest possible strain is applied to the conductor (generally the strain is adjusted to within 0.1%).
FIG. 1 shows an exemplary structure of the cable-in-conduit type superconductor. The superconductor is basically composed of a conduit into which is inserted a cable that consists of superconductor strands (optionally together with copper wires).
When a time-varying magnetic field is applied to the superconductor, an induced current called coupling current flows between strands. Since the coupling current flows through the normal conducting portions (the stabilized copper portion and the plate on the superconductor strands), Joule's heat is generated to cause loss (ac loss). In order to reduce the ac loss, the resistance between strands may be increased so that the coupling loss decreases. The ac loss Qc is generally related to the inter-strand resistance ρ by the following expression:Qc∝1/ρ  (1)and Qc can be reduced by increasing ρ.
On the other hand, in order to ensure that a current flows uniformly through a plurality of superconductor strands, they must have a certain degree of conductivity between themselves. To this end, a suitable value of inter-strand resistance is provided by plating the superconductor strands with chromium, nickel, etc. In particular, niobium, tin or niobium-aluminum strands that need heat treatment to form superconducting compounds frequently use chromium as a highly heat-resistant plate material. However, the chromium plate sinters during heat treatment of the conductor and this is considered to cause a marked decrease in inter-strand resistance.
It is generally known that the critical current characteristic of superconductor strands deteriorates if they are subjected to strain. In order to avoid this problem, superconducting coil manufacturers have been careful to ensure that the smallest possible strain is applied to the superconductor after heat treatment (the strain being typically adjusted to within 0.1%). As a consequence, most of the sintered plate on the superconducting strands remains intact and the inter-strand resistance drops to increase the ac loss of the superconductor.